1. Field of the Invention
The present invention relates to oxide type superconducting materials. More particularly, it relates to a novel superconducting material of oxide having a higher critical temperature and a high critical current density.
2. Description of the Related Art
Since Bednorz and Muller discovered a compound oxide type superconductor of La--Sr--Cu system in 1986 [Z. Phys. B64 (1986) 189], new types of superconducting materials which have much higher Tc than the conventional superconductors have been reported successively.
In 1987, it was reported that C. W. Chu et al. discovered, in the United States of America, another superconducting material so called Y--Ba--Cu system having the critical temperature of about 90 K which is higher than the boiling point 77 K of liquid nitrogen (Physical Review letters, Vol. 58, No. 9, p 908).
Another type new superconducting material reported by Maeda et al. in Japan is a compound oxide of Bi--Sr--Ca--Cu system which have a critical temperature of higher then 100 K (Japanese Journal of Applied Physics. Vol. 27, No. 2, p 1209 to 1210).
Still another type new superconducting material reported by Hermann et al. in the United States of America is a compound oxide of Tl--Ba--Ca--Cu system which have a critical temperature of higher then 120 K. (Appl. Phys. Lett. 52 (20) p 1738).
The critical temperatures of the Bi-type superconductors and the Tl-type superconductors are fairly high than the boiling point of liquid nitrogen (77 K) and hence they are expected to be applicable in the actual utilization of high Tc superconductors.
These superconductors, however, are obtained as mixed phases: one phase showing the critical temperature of higher than 100 K and another phase showing the critical temperature of about 80 K and it is not easy to produce a compound oxide composed of a single phase of the high Tc alone, so that several ideas to produce a compound oxide consisting of the high Tc single phase alone have been proposed.
In the case of Bi--Sr--Ca--Cu system, it has been proposed to add lead (Pb) to the compound oxide of this system in order to increase the contents of the higher Tc phase. Although this solution is effective to improve the critical temperature, this process requires very long sintering time and the critical current density of the resulting compound oxide of Bi--Pb--Sr--Ca--Cu system is very low.
The compound oxide of Tl--Ba--Ca--Cu system contains several superconducting phases including Tl.sub.2 Ca.sub.2 Ba.sub.2 Cu.sub.3 O.sub.x and Tl.sub.2 CaBa.sub.2 Cu.sub.2 O.sub.y or the like. Although this superconductor shows a very high critical temperature of about 120 K, it is difficult to produce a compound oxide consisting of a single phase of the high Tc alone, because it is not easy to control precisely the composition of elements in a product because thallium (Tl) is a very volatile element due to its high vapour pressure. Still more, thallium is very toxic for human.
Therefore, an object of the present invention is to provide a novel superconducting material of compound oxide which shows almost same high level of critical temperature as the known Tl-system compound oxide and which can be produced stably.